For example, as shown in FIG. 1, with a rectangular plate (a/b≧2.5, a>>L, b>>L) having an aspect ratio (a/b) of 2.5 or more, the electromechanical coupling factor k31 in the lateral vibration mode is proportional to the square root of the conversion efficiency of electric energy or mechanical energy for the magnitude of vibration (lateral vibration) in a direction 1 orthogonal to a polarization direction 3 at the time of applying a voltage in the polarization direction 3. The higher this value, the more the efficiency improves. It should be noted that the shape of a piezoelectric single crystal device may be a square shape, a disc shape, a rod shape, or the like, instead of the above-described rectangular plate shape, and the electromechanical coupling factor k31 can be determined in the same way with any one of these shapes.
In general, lead zircon titanate Pb(Zr, Ti)O3 (PZT) disclosed in T. Ogawa, M. Matsushita, Y. Tachi, and K. Echizenya, “Program Summary and Extended Abstracts of the 10th US-Japan Seminar on Dielectric and Piezoelectric Ceramics” (September 26-29, (2001), pp. 245-248) is widely used as a material for forming the piezoelectric device. However, lead zircon titanate (PZT) described in the document of Ogawa et al has an electromechanical coupling factor k31 of about 30%.
In order to obtain higher k31 than that of the above-described PZT, for example, Japanese Unexamined Patent Application Publication No. 11-171644 discloses a piezoelectric ceramic composition containing x(Pb2Me2O7)½·(1-x)[Pb(Zr1-yTiy)O3] as a main component and Cr and Si secondary components. However, the piezoelectric ceramic composition disclosed in Japanese Unexamined Patent Application Publication No. 11-171644 has an electromechanical coupling factor k31 of 40% or less.
Furthermore, Jpn. J. Appl. Phys. 90 (2001) (pp. 3471-3475) discloses the measured piezoelectric properties of a single crystal of 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 having an electromechanical coupling factor k31 of as high as 59% (0.59), such as lateral mode k31 in the [100] or [010] direction orthogonal to the [001] direction as the polarization direction.
However, Jpn. J. Appl. Phys. 90 (2001) (pp. 3471-3475) discloses the electromechanical coupling factor k31 measured at room temperature, but the electromechanical coupling factor k31 in use in a high-temperature (specifically 150° C.) environment is not disclosed. For example, in use as a piezoelectric device, the piezoelectric device may be soldered or bonded to a resin and thus used in an environment in which the temperature changes from room temperature to a high temperature (specifically 150° C.). In this case, the piezoelectric device tends to be decreased in electromechanical coupling factor k31 due to deterioration. This point is not disclosed or suggested in Jpn. J. Appl. Phys. 90 (2001) (pp. 3471-3475).